1. Field of the Invention
The present invention relates to a technology that is applied to generate a look-up table (LUT) for performing conversion processing on color data in an extended color space.
2. Description of the Related Art
A look up table is used to convert input color space data into output color space data. Normally, since the input color space has a great number of gradation levels, the LUT does not have the conversion values for all color data in the input color space, but prepares the conversion values for a plurality of representative grid points therein. For example, when the input color space has ‘m’ dimensions and n grid points are set for each channel, the LUT has the grid point data of the m-th power of n (nm). For input color space data which is not at a grid point, an interpolation calculation is performed using data values of surrounding grid points to calculate output color space data corresponding to the input color space data.
In recent years, as a standard RGB color space, an scRGB (extended color space) is defined by extending a sRGB (base color space). In the scRGB color space, a range of color values for respective channels of red component (R), green component (G), and blue component (B) is extended from color values between 0.0 and 1.0 to color values between −0.5 and 7.5. Further, the number of bits for each channel is increased from 8 bits to 16 bits. With these extensions, the data in a sRGB color space (color values between 0.0 and 1.0) before performing the extension can be treated as it is.
A technology of a look up table applied to an extended color space is discussed in U.S. Pat. No. 7,034,844 and Japanese Patent Application Laid-Open No. 2006-093915.
When a look up table corresponding to an extended color space is generated using a method similar to a technology described in the above-described documents, the data size of the look up table is increased.
For example, when an sRGB color space (color values between 0.0 and 1.0) is extended to an scRGB color space (color values between −0.5 and 7.5) without changing the distance between grid points in the LUT, a range of an input value of the look up table needs to be increased up to eight times. When the look up table of the sRGB color space has 9 grid points, the scRGB color space has 65 grid points. When the look up table of the sRGB color space has 17 grid points, the scRGB color space has 129 grid points. When the look up table of the sRGB color space has 33 grid points, the scRGB color space has 257 grid points. Ratios of look up table sizes between the sRGB color space and the scRGB color space are respectively 653/93=376.7 times, 1293/173=436.9 times, and 2573/333=472.3 times.
Even if the range of the scRGB color space is limited to the color values between −0.5 and 1.5, the ratios of the look up table sizes are respectively 173/93=6.7 times, 333/173=7.3 times, and 653/333=7.6 times.